RF Amplifier Architecture and Related Techniques

ABSTRACT

A radio frequency (RF) power amplifier system or transmitter includes one or more power amplifiers and a controller that is configured to adjust amplitudes and phases of RF input signals of the one or more power amplifiers and supply voltages applied to the one or more power amplifiers. In embodiments where multiple power amplifiers are used, a combiner may be provided to combine outputs of the power amplifiers. In at least one implementation, amplitude adjustment of the RF input signals of the one or more power amplifiers may be used to provide transmit power control and/or power backoff.

FIELD

Subject matter disclosed herein relates generally to radio frequency(RF) circuits and, more particularly, to devices, systems, andtechniques for implementing RF amplifiers and transmitters that arecapable of achieving high linearity and high efficiency simultaneously.

BACKGROUND

As is known in the art, a radio frequency (RF) transmitter is a devicethat produces RF signals. RF transmitters may be included, for example,as part of a radio communication system that uses electromagnetic waves(radio waves) to transport information over a distance.

As is also known, a trade-off must generally be made in RFcommunications transmitters between energy efficiency and linearity.Over the decades of development of the RF transmitter and correspondingRF amplifiers, it has generally been true that one could obtain eitherhigh efficiency or high linearity, but not both. It would, therefore, bedesirable to provide systems and techniques that allow a user to amplifyRF signals and/or transmit data carrying RF signals with both highefficiency and high linearity.

SUMMARY

Systems and techniques are described herein that allow radio frequency(RF) signals to be amplified and/or transmitted with both highefficiency and high linearity. Digital control may be maintained overboth the amplitudes and the phases of RF input signals applied to one ormore power amplifiers. Digital control may also be maintained over thesupply voltages applied to the one or more power amplifiers. In someembodiments, non-linear power amplifiers may be used to achieve highpower efficiency within the RF transmitter. Digital control techniquesmay be used to achieve linearity and to further enhance efficiency. Inat least one implementation, the amplitudes of one or more RF inputsignals of the one or more power amplifiers may be controlled to achievebackoff in the power amplifier to generate a desired output power level.

In accordance with one aspect of the concepts, systems, circuits, andtechniques described herein, an RF power amplifier system comprises: anRF power amplifier; a digital-to-RF modulator to generate an RF inputsignal for the RF power amplifier based on input information indicativeof amplitude values and phase values; a voltage control unit to providea variable supply voltage to the RF power amplifier in response to acontrol signal, the variable supply voltage being selected from aplurality of discrete voltage levels; and a controller to provide theinput information to the digital-to-RF modulator and the control signalto the voltage control unit based, at least in part, on data to betransmitted from the RF transmitter, the controller being configured tomake decisions about voltage level changes for the RF power amplifierbased, at least in part, on a window of data samples representing datato be transmitted by the RF transmitter.

In accordance with another aspect of the concepts, systems, circuits,and techniques described herein, a machine implemented method foroperating an RF transmitter having a digital-to-RF modulator driving apower amplifier comprises: obtaining transmit data to be transmittedfrom the RF transmitter; providing input information for thedigital-to-RF modulator based, at least in part, on the transmit data,the input information to control an amplitude and a phase of an RFoutput signal of the digital-to-RF modulator; and selecting a supplyvoltage for the power amplifier based, at least in part, on the transmitdata, wherein selecting a supply voltage includes making decisions aboutvoltage level changes for the RF power amplifier based, at least inpart, on a window of data samples representing data to be transmitted bythe RF transmitter.

In accordance with still another aspect of the concepts, systems,circuits, and techniques described herein, an apparatus comprises acomputer readable medium having Instructions stored thereon that, whenexecuted by a computer, perform a method for operating an RF transmitterhaving a digital-to-radio frequency (RF) modulator driving a poweramplifier, the method comprising: obtaining transmit data to betransmitted from the RF transmitter providing input information for thedigital-to-RF modulator based, at least in part, on the transmit data,the Input information to control an amplitude and a phase of an RFoutput signal of the digital-to-RF modulator; and selecting a supplyvoltage for the power amplifier based, at least in part, on the transmitdata, the supply voltage being selected from a plurality of discretevoltage levels, wherein selecting a supply voltage includes making adecision about supply voltage level for the RF power amplifier based, atleast in part, on a window of data samples representing data to betransmitted by the RF transmitter.

In accordance with a further aspect of the concepts, systems, circuits,and techniques described herein, a machine implemented method forselecting a voltage level for a power amplifier within a radio frequency(RF) transmitter based on input data to be transmitted comprises:identifying a window of samples to be transmitted from the transmitter;and if all samples in the window of samples can use a lower voltagelevel than a previous sample to achieve a desired instantaneous outputpower, selecting a new voltage level that is lower than that used forthe previous sample.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features may be more fully understood from the followingdescription of the drawings in which:

FIG. 1 is a block diagram Illustrating an exemplary radio frequency (RF)transmitter in accordance with an embodiment;

FIG. 2 is a block diagram Illustrating an exemplary RF transmitter inaccordance with another embodiment;

FIG. 3 is a block diagram illustrating an exemplary RF transmitterhaving at least four power amplifiers in accordance with an embodiment;

FIG. 4 is a block diagram illustrating an exemplary RF transmitterhaving a single power amplifier in accordance with an embodiment;

FIG. 4A is a block diagram illustrating an exemplary RF transmitterhaving a single power amplifier that uses a discrete voltage switchingin accordance with an embodiment;

FIG. 4B is a schematic diagram illustrating an exemplary switchedcapacitor converter circuit that may be used within a voltage controlunit in accordance with an embodiment;

FIG. 4C is a schematic diagram illustrating a voltage controlarrangement for switching among different discrete supply levels for apower amplifier in accordance with an embodiment;

FIGS. 4D and 4E are schematic diagrams illustrating exemplary switchingnetwork architectures that may be used in power amplifier systems and RFtransmitter systems in accordance with embodiments;

FIGS. 4F-4I are schematic diagrams illustrating exemplarylevel-transition filter architectures that may be used in variousembodiments;

FIG. 5 is a block diagram illustrating an exemplary RF transmitter inaccordance with another embodiment;

FIG. 6 is a schematic diagram Illustrating an exemplary voltage controlunit in accordance with an embodiment;

FIG. 7 is a schematic diagram illustrating an exemplary voltage controlunit in accordance with another embodiment;

FIG. 8 is a flow diagram illustrating a method for operating an RFtransmitter having at least two digital-to-RF modulators driving atleast two power amplifiers in accordance with an embodiment;

FIG. 9 is a flow diagram Illustrating a method for operating an RFtransmitter having a single digital-to-RF modulator driving a singlepower amplifier in accordance with an embodiment;

FIG. 10 is a diagram Illustrating an exemplary sample window that may beused to make a voltage level change decision in a power amplificationsystem in accordance with an embodiment; and

FIG. 11 is a flow diagram Illustrating a method for selecting voltagelevels for one or more power amplifiers of a power amplification systembased on a sample window in accordance with an embodiment;

FIG. 12 is a plot Illustrating the efficiency and output power of apower amplification system having a single power amplifier as a functionof normalized RF drive power at two different dc power supply levels;

FIG. 13 is a pair of IQ plots Illustrating achievable RF outputamplitudes for a power amplification system having a single poweramplifier using two different supply levels;

FIG. 14 is a plot of efficiency versus normalized output power for apower amplification system having a single power amplifier using twodifferent supply levels in accordance with an embodiment;

FIG. 15 is an IQ plot Illustrating achievable output signal ranges for apower amplification system having two power amplifiers and a combiner,where each power amplifier is supplied from one of two supply levels;

FIG. 16 is an IQ plot Illustrating operation of an exemplary AsymmetricMulitlevel Outphasing (AMO) based amplification system that includestwo-power amplifiers and two power supply levels;

FIG. 17 is an IQ plot illustrating a control technique that may be usedwith a power amplification system having two power amplifiers and acombiner to operate the power amplifiers at or close to their achievablemaximum amplitudes, while minimizing output at an isolation port of thecombiner; and

FIG. 18 is a series of plots illustrating how power amplifier drivelevels and supply levels may be adjusted to control output power over anoutput power range in accordance with an embodiment.

DETAILED DESCRIPTION

FIG. 1 is a block diagram illustrating an exemplary radio frequency (RF)transmitter 10 in accordance with an embodiment. As will be described ingreater detail, RF transmitter 10 is capable of simultaneously achievingboth high efficiency and high linearity. As illustrated, RF transmitter10 may include: a controller 12; first and second digital to RFmodulators 14, 16; first and second power amplifiers 18, 20; first andsecond voltage control units 22, 24; a power combiner 26; and an energyrecovery module 30. RF transmitter 10 may be coupled to one or moreantennas 32 and/or other transducers to facilitate the transmission ofRF signals to one or more remote wireless entities. In someImplementations, first and second power amplifiers 18, 20 may usesubstantially the same amplifier design. In other implementations,different amplifier designs and/or architectures may be used. In someembodiments, first and second power amplifiers 18, 20 may be non-linearamplifiers (and, in some cases, highly non-linear amplifiers). As iswell known, non-linear amplifiers generally operate more efficientlythan linear amplifiers.

First and second digital to RF modulators 14, 16 are operative forgenerating RF input signals for first and second power amplifiers 18,20, respectively, based on information received from controller 12.First and second voltage control units 22, 24 are operative forproviding variable supply voltages to first and second power amplifiers18, 20, respectively, based on control signals received from controller12. In a typical implementation, controller 12 will receive or otherwiseobtain a stream of data to be transmitted from RF transmitter 10 (i.e.,transmit data). Controller 12 may then use this transmit data, amongother things, to provide signals for first and second digital to RFmodulators 14, 16 and first and second voltage control units 22, 24 thatwill result in the transmission of the transmit data from antenna(s) 32.Controller 12 may update the information delivered to first and seconddigital to RF modulators 14, 16 and the control signals delivered tofirst and second voltage control units 22, 24 on a sample-by-samplebasis in some implementations.

In at least one implementation, controller 12 can provide Independentcontrol to each of first and second voltage control units 22, 24 so thatdifferent supply voltages can be simultaneously applied to first andsecond power amplifiers 18, 20. Likewise, in some implementations,controller 12 can provide different input Information to first andsecond digital to RF modulators 14, 16 so that different RF inputsignals are simultaneously applied to first and second power amplifiers18, 20. Power combiner 26 is operative for combining the output signalsof first and second power amplifiers 18, 20 to generate an RF transmitsignal at an output thereof. The RF transmit signal may then bedelivered to antenna(s) 32 for transmission into a wireless channel. Aswill be appreciated, the RF transmit signal should include an accuraterepresentation of the original transmit data.

First and second power amplifiers 18, 20 may each include any type ofpower amplifier capable of amplifying an RF signal. In someimplementations, first and second power amplifiers 18, 20 may benon-linear amplifiers to improve the efficiency of operation of the RFtransmitter. In some embodiments, highly non-linear amplifiers may beused. First and second power amplifiers 18, 20 may use the sameamplifier design or different amplifier designs. Likewise, first andsecond power amplifiers 18, 20 may use the same amplifier architectureor different amplifier architectures. First and second voltage controlunits 22, 24 may each include any type of circuit, component, or systemthat is capable of controllably varying a supply voltage level appliedto a power amplifier in an RF transmitter. These units may include, forexample, variable power supplies, discrete power supplies, batteries,multi-level power converters, and/or switching circuits that are capableof switching between preset voltage potentials.

First and second digital to RF modulators 14, 16 may include any type ofcircuits or components that are capable of converting digital inputinformation representative of a time varying amplitude and atime-varying phase into an analog RF output signal having correspondingamplitude and phase characteristics. Power combiner 26 may include anytype of device or structure that is capable of combining multiple RFsignals. This may include, for example, a hybrid combiner, a transformercombiner, a Wilkinson combiner, and or others. Power combiner 26 may bean isolating combiner or a non-isolating combiner.

As described above, controller 12 may receive or otherwise acquiretransmit data that needs to be transmitted into a wireless channel. Thetransmit data may be in any format (e.g., a binary bit stream; I and Qdata; etc.). Controller 12 may then use this data, as well as otherpossible factors, to provide signals for first and second digital to RFmodulators 14, 16 and first and second voltage control units 22, 24. Insome implementations, the goal may be to generate an RF transmit signalthat includes an accurate representation of the transmit data. Any of anumber of different modulation and coding schemes (MCSs) may be used torepresent the transmit data within the RF transmit signal. The MCS mayinclude, for example, binary phase shift keying (BPSK), quadrature phaseshift keying (QPSK), quadrature amplitude modulation (e.g., QAM, 16 QAM,64 QAM, 128 QAM, etc), orthogonal frequency division multiplexing(OFDM), and/or others. Some of these MCSs have relatively high peak toaverage power ratios. As is well known, MCSs having high peak to averagepower ratios typically require highly linear power amplification toprovide an accurate representation of transmit data. In variousembodiments described herein, transmission systems and techniques aredescribed that are capable of providing efficient power amplificationwith sufficient linearity to support MCSs having high peak to averagepower ratios.

As shown in FIG. 1, after controller 12 acquires transmit data, it mayuse the data to provide input Information to first and second digital toRF modulators 14, 16. In one possible approach, controller 12 mayprovide separate I and Q data to each of the first and second digital toRF modulators 14, 16. That is, controller 12 may generate I₁, Q₁, forfirst digital to RF modulator 14 and I₂, Q₂ for second digital to RFmodulator 16. First and second digital to RF modulators 14, 16 may thenuse the I, Q information to modulate an RF carrier wave to generate acorresponding RF signal at an output thereof. As is well known, I and Qdata is generally representative of an amplitude and a phase. Thus, I₁and Q₁ may, for example, have a corresponding amplitude A₁, and phaseθ₁. The RF signal output by first digital to RF modulator 14 in responseto I₁ and Q₁ may therefore be an RF signal having amplitude A₁ and phaseθ₁. In some implementations, the input information provided to first andsecond digital to RF modulators 14, 16 may be in a format other than Iand Q. For example, in one possible approach, amplitude (A1, A2) andphase (θ₁, θ₂) information may be delivered to first and second digitalto RF modulators 14, 16 by controller 12. As described above, the inputinformation applied to first and second digital to RF modulators 14, 16may change on a sample by sample basis in some embodiments.

In some implementations, first and second voltage control units 22, 24may each be capable of providing one of a plurality of predeterminedvoltages to corresponding power amplifiers 18, 20 in response to controlsignals from controller 12. Thus, a control signal V_(CONT1) may selecta voltage value for power amplifier 18 and a control signal V_(CONT2)may select a voltage value for power amplifier 20. As with the inputinformation applied to first and second digital to RF modulators 14, 16,the supply voltage values applied to first and second power amplifiers18, 20 may change on a sample by sample basis in some embodiments.

In addition to the above, in some embodiments, controller 12 may use theamplitude information delivered to first and second digital to RFmodulators 14, 16 (e.g., the amplitude value associated with I₁ and Q₁,etc.) to control/adjust a power level output by the RF transmitter 10(e.g., a transmit power level). For example, controller 12 may use areduced amplitude value for one or both of the digital to RF modulators14, 16 when a lower transmit power level is desired.

In some embodiments, controller 12 may determine input information forfirst and second digital to RF modulators 14, 16 and control Informationfor first and second voltage control units 22, 24 in real time based onthe transmit data. In some other embodiments, an optional lookup table(LUT) 36 may be used to provide the required information. Controller 12may retrieve values from LUT 36 for the first and second digital to RFmodulators 14, 16 and the first and second voltage control units 22, 24that are designed to accurately represent the transmit data in thetransmit signal.

In some embodiments, power combiner 26 may be an isolating combinerhaving an isolation port. As is well know, an isolating combiner willsometimes output energy at the isolation port due to, for example,mismatches, imbalances, and/or reflections in the circuitry coupled tothe combiner. Typically, a resistive termination will be coupled to theisolation port of an isolating combiner to provide an impedance matchfor the port and to dissipate any energy output from the port. In someembodiments, an energy recovery module 30 may be coupled to theisolation port of an isolating combiner, rather than a conventionalresistive termination, for use in recovering some or all of the energythat would otherwise have been dissipated. Energy recovery module 30 mayinclude circuitry for converting the recovered energy into a usefulform. For example, the recovered energy may be converted to a form thatcan be used to charge a battery. Alternatively, the recovered energy maybe converted to a form that may be used to energize other circuitrywithin RF transmitter 10.

In the embodiment illustrated in FIG. 1, separate voltage control units22, 24 are provided for first and second power amplifiers 18, 20. Insome implementations, the functions of multiple voltage control unitsmay be implemented using a single voltage control structure. FIG. 2 is ablock diagram Illustrating an exemplary radio frequency (RF) transmitter40 in accordance with an embodiment. As shown in FIG. 2, a singlevoltage control unit 34 may be used to provide variable voltages forboth of power amplifiers 18, 20.

In some embodiments, power amplification systems are provided thatincorporate dynamic selection from among a discrete set of voltagelevels for providing drain bias voltages to a set of power amplifiers(PAs). Selection from among multiple discrete voltage levels in settingone or more drain bias voltages is common to a number of systemarchitectures. This includes systems that select from among a discreteset of input voltages and then provide additional regulation to providea continuously-varying drain voltage (see, e.g., “Multilevel PowerSupply for High Efficiency RF Amplifiers,” by Vasic et al., 2009 IEEEApplied Power Electronics Conference, pp. 1233-1238, February 2009; andU.S. Pat. No. 7,482,869 to Wilson, entitled “High EfficiencyAmplification.”) and systems that directly exploit discrete drainlevels, including “class G” amplifiers (see, e.g., “Average Efficiencyof Class-G Power Amplifiers,” by F. H. Raab, IEEE Transactions onConsumer Electronics, Vol. CE-32, no. 2, pp. 145-150, May 1986; and “AClass-G Supply Modulator and Class-E PA in 130 nm CMOS,” by Walling etal., IEEE Journal of Solid-State Circuits, Vol. 44, No. 9, pp.2339-2347, September 2009), multi-level LINC (MLINC) Power Amplifiers(see, e.g., U.S. Patent Application Publication 2008/0019459 to Chen etal. entitled “Multilevel LINC Transmitter,” and U.S. Patent ApplicationPublication US 2010/0073084 to Hur et al. entitled “Systems and Methodsfor a Level-Shifting High-Efficiency LINC Amplifier using Dynamic PowerSupply” and Asymmetric Multilevel Outphasing (AMO) Power Amplifiers(see, e.g., U.S. Pat. Nos. 8,026,763 and 8,164,384 to Dawson et al.entitled “Asymmetric Multilevel Outphasing Architecture for RFAmplifiers” and “A 2.4-GHz, 27-dBm Asymmetric Multilevel OutphasingPower Amplifier in 65-nm CMOS” by Godoy at al., IEEE Journal ofSolid-State Circuits, 2012). In the present application, dynamicselection from among a discrete set of voltage levels (or “levelselection”) is primarily discussed in the context of a single ormulti-amplifier, multi-level power amplifier architecture (such as, forexample, the power amplifier architecture used in RF transmitter 10 ofFIG. 1). However, the level selection techniques discussed herein canalso be used to improve performance in other architectures utilizingdiscrete voltage levels, including those enumerated above.

Level selection can be performed based upon the instantaneous outputenvelope value or, when processing digital data from which to synthesizethe RF output, upon an individual digital sample to be synthesized. Inone possible approach, the lowest discrete voltage level or levels maybe used that are compatible with synthesizing the required InstantaneousRF output in order to provide the highest instantaneous “drain”efficiency. In at least one embodiment, discrete voltage levels may beselected based upon a “window” of data to be transmitted, rather than asingle sample or “instantaneous” value. In this manner, the number oflevel transitions that are used to synthesize a particular data streamcan be reduced. In many cases, this technique may result in highervoltage levels being used than are absolutely necessary to synthesize aparticular sample (thus reducing power amplifier “drain” efficiency).However, overall efficiency can be improved (or at least its degradationreduced) by mitigating the energy cost of transitioning among levels.Moreover, linearity (e.g., as measured by adjacent channel leakage ratio(ACLR) or adjacent channel power ratio (ACPR)), error vector magnitude(EVM), and other waveform quality metrics can be improved by reducingthe number of level transitions required. This is because the systemdisturbance caused by transitioning levels on one or more PAs willtypically have a cost in terms of the output trajectory, even thoughthis can be compensated for using digital pre-distortion or othertechniques.

In some embodiments, decisions may be made about the discrete voltagelevels to apply based on one or more extended time periods, rather thanrelying solely on Instantaneous conditions. In one approach, forexample, a decision as to whether or not to change selected level(s)looks across a window of samples N_(W) long (e.g., from a current sampleunder consideration into the future, from a previous sample in the pastthrough the current sample and for one or more samples into the future,etc.). FIG. 10 is a diagram Illustrating such a sample window 220. Thesesamples may be represented as I, Q data values, as amplitudes/phases ofthe desired outputs, as vector values to be synthesized, etc., or maysimply represent the output power or voltage amplitudes desired. For thecurrent sample 222, the system may only transition to a lower level orset of levels (i.e., providing a lower maximum power) if that lowerlevel or set of levels can provide sufficient output power over thewhole window 220 of N_(w) samples. If more power delivery is needed forthe current sample 222 than can be provided by the level/set of levelsused in the previous sample 224, then a transition is made to alevel/set of levels capable of providing sufficient power. Otherwise,the level/set of levels remains unchanged from the previous sample 224to the current sample 222.

Using the above-described technique reduces the number of transitions byensuring that a “downward” level transition only occurs if an “upward”transition won't be necessary again in the near future. At the sametime, an “upward” transition may always be used if needed to support therequired output power during a given sample. It will be appreciated thatmany variations are possible which provide further reduction in numberof transitions. For example, in one alternative approach, a decision maybe made to make either an “upward” or a “downward” transition as long asit is certain that another transition will not be needed for at least aminimum number of samples (i.e., within a window of samples). In anotheralternative approach, if an “upward” transition is needed to support anoutput for less than a certain number of samples, that transition couldbe eliminated, and the power amplifiers driven to provide an output asclose as possible to the desired output (during either the sample inquestion or averaged in some manner over multiple samples). This mayreduce transitions, at the expense of some waveform quality degradation.If any of the above-described techniques are used, the input drives forthe individual power amplifiers may be adjusted accordingly for theselected drain voltage levels in order to provide the desired totaloutput. In some embodiments, a controller may be programmed orconfigured to be capable of selecting a voltage value for a poweramplifier that is lower than required to generate a desiredinstantaneous output power level if the need for a higher voltage levelis short in duration compared to the time duration of the window of datasamples. In other embodiments, a controller may be programmed orconfigured to be capable of selecting a voltage value for a poweramplifier that is higher than a minimum voltage sufficient to generate adesired instantaneous output power level if the duration of time forwhich a lower voltage level is sufficient is short compared to the timeduration of the window of data samples.

In some implementations, the above-described technique may be combinedwith other techniques including, for example, peak-to-average powerratio (PAPR) reduction techniques, techniques using hysteresis in thethresholds used to make level selection determinations, techniques thatfilter the data before and/or after the level-selection process, and/orothers. In some implementations, the window lengths that are used withthe above-described techniques may be dynamically and/or adaptivelyselected based on a performance criterion. In one approach, for example,the window lengths may be selected to maximize a combination ofefficiency and output quality metrics.

To provide a better understanding of the operation and benefits of theamplifier architectures described herein, the performance of a singlepower amplifier (PA) having two different dc supply voltages will now bedescribed. This power amplifier could represent any number of PA classes(e.g., A, AB, B, F, Inverse F, etc.) and could also represent acomposite amplifier comprising an aggregation of output from multiplesmaller PAs operated together (e.g., a Doherty amplifier, outphasingamplifier, etc.) FIG. 12 is a plot showing the efficiency and(normalized) output power of a power amplifier as a function ofnormalized RF drive power at two different dc power supply levels (i.e.,a lower dc supply, level 1, and a higher dc supply, level 2). It can beseen that for the level 2 dc supply, at a certain RF input power (0.1normalized) the output power reaches a maximum saturated output power (1normalized). The output power can be reduced to any value below thissaturated maximum (i.e., “backing off” the output power) by reducing theRF Input power. For low levels of RF input and output power, there is anearly linear (proportional) relation between RF input power and RFoutput power. However, efficiency in this region is relatively low(e.g., below 40% for RF output powers below 0.25 normalized). Thehighest efficiency is found in regions of output power at or somewhatbelow the level at which output power saturates (e.g., efficiency above70% for output power above 0.83 normalized). Increasing RF input powerbeyond the level that saturates the power amplifier, however, actuallyreduces efficiency. This occurs because total input power—dc plusRF—increases but output power does not increase (and in some cases candecrease with further increase in RF input power).

The power and efficiency for the lower voltage dc supply (level 1) isnow considered. In this case, the maximum saturated output power is muchlower than for the higher-voltage (level 2) dc supply (e.g., reaching amaximum output power of only 0.25 normalized). The output power canagain be adjusted between zero and this lower maximum value by adjustingRF input power (e.g., backing off the output power by reducing thenormalized input power to values below that which saturates the poweramplifier), but higher output powers (above 0.25 normalized) are notobtainable at this supply level. It should be noted that for values ofoutput power that can be reached at this lower supply level, higherefficiency is achieved using the lower dc supply level 1 than for thehigher dc supply level 2, because the power amplifier is operated closerto its saturated value. Thus, for low values of output power, it isgenerally desirable to use a lower supply voltage value, so long as thedesired output power is achievable and the desired level of linearityand controllability of the power amplifier is achievable.

The variation in efficiency with a given dc voltage supply level is amotivation for power amplifier systems such as “Class G,” that switchthe power amplifier dc supply among different levels depending on thedesired RF output power level. Selecting from multiple dc supply valuessuch that the power amplifier operates at as high an efficiency aspossible while being able to provide the desired RF output power canyield significant improvements in efficiency over that achieved with asingle supply level.

FIG. 13 includes two IQ plots illustrating the achievable RF outputs interms of the output RF amplitude (i.e., phasor length, or RF voltageamplitude) for two different supply levels. For a given supply level,there is an RF output amplitude (proportional to the square root of RFoutput power) that may be specified as a maximum for that supply level.This maximum amplitude may be that corresponding to the absolute maximumsaturated output power (under complete compression) for that supplylevel, as illustrated in FIG. 12, or may be a level somewhat below this.One may limit the maximum amplitude and power to somewhat slightly lowerthan those for complete saturation to simplify predistortion of drivesignals (for linearization), to account for part-to-part variations inabsolute maximum power, to place the specified maximum level in adesirable location on the efficiency vs. output characteristic, or forother reasons. As illustrated in FIG. 13, with the higher supply level,any RF output voltage vector having an amplitude less than or equal tothe radius of the circle labeled L2 can be synthesized. With the lowersupply level, one can synthesize any RF output voltage vector having anamplitude less than or equal to the radius of the circle labeled L1.

To exploit the availability of multiple supply voltages to achieveincreased efficiency, one may dynamically switch between the two supplylevels. One way to do this is to switch supply levels based on theamplitude of the RF output vector being synthesized at any given time,such that the higher supply level is utilized whenever the desiredoutput amplitude is between L1 and L2, and the lower supply levelwhenever the desired output amplitude is at or below L1. Using thisapproach, leads to the efficiency vs. normalized output powercharacteristic shown in FIG. 14.

In at least one embodiment, the signal to be synthesized is consideredover a longer interval (e.g., a window including multiple future digitalsamples) and level switching is managed based on the moving window ofdata. For example, this may be done in a way that ensures that thedesired instantaneous output amplitude can always be synthesized, butswitch down to the lower supply level only if the desired output signalamplitude will remain at a level at or below L1 for a minimum duration.Amplitude or time hysteresis or other constraints can likewise be putinto level switching decisions. For example, one may require that once aswitching transition is made, no other transition will be made for aspecified interval. Moreover, while this is Illustrated for two powersupply levels, the approach may be easily extended to an arbitrarynumber of supply levels. The length of the window used could bepreselected, or could be dynamically selected based on one or more of arange of factors including output power levels (short or long term),channel characteristics, data bandwidth, signal statistics, transmitdata, etc. Likewise, one could select the power supply levels based onthe contents of a block of samples at a time, where the block length isfixed or dynamically selected.

An important consideration in the above discussion is that instantaneousefficiency is higher for operating points where the length of thesynthesized vector is dose to the maximum achievable amplitude radius.From this perspective, it is desirable to operate from the lowest supplyfor which the desired output can be synthesized. However, as there is anenergy cost to switching between levels (e.g., owing to capacitorcharging loss and switching loss of transistors setting the supply), itmay be beneficial to overall system efficiency to use ahigher-than-necessary supply level to synthesize an output over a shortduration, if the energy cost of switching back and forth between levelsis greater than that saved by using a lower supply level over thatduration. Moreover, there is also a cost to linearity associated withswitching among levels, in terms of noise injected to the output andloss of linearity owing to temporary disturbance of the power amplifiereach time a level transition is made. Even if there is an overallefficiency penalty, one may choose to operate from a higher supply levelthan necessary to synthesize a particular output over a short durationif it reduces the number of supply transitions necessary, in order toimprove linearity. One may also choose not to briefly step up a powersupply level for a short duration, even if the desired instantaneouspower does not get synthesized (possibly briefly clipping the peak ofthe signal), owing to the loss and noise injection penalty of switchingbetween levels. The impact of level transitions on both efficiency andlinearity may thus be considered as factors in selecting what level touse in a particular interval.

Further advantage may be attained in some embodiments by combining powerfrom multiple power amplifiers (see, e.g., system 10 of FIG. 1). In onemultiple power amplifier embodiment, there may be, for example, twopower amplifiers that can each be supplied from two different (nonzero)supply voltages and an isolating power combiner to combine the outputsignals of the amplifiers in an isolating fashion. The voltage vector atthe output port of the power combiner is a weighted sum of the outputvoltage vectors of the two individual power amplifiers. Likewise, a2-way isolating combiner may produce an isolation voltage vector at thecombiner isolation port that is a weighted difference of the twoindividual power amplifiers. The total energy delivered by the poweramplifiers to the combiner is ideally delivered to either the outputport (e.g., to be transmitted) or to the isolation port (e.g., where itmay be dissipated in an isolation resistor, recovered via an energyrecovery system, or transmitted via a second transmission path). Forexample, if the output phasor vectors of the individual PAs are V₁, V₂,the output port of the power combiner may produce a sum vector:

Σ=V _(out) =V ₁ +V ₂

and the isolation port of the combiner may produce a difference vector:

Δ=V _(iso) =V ₁ −V ₂

As is known in the art, for an isolating combiner, the input impedancesat the combiner inputs and the impedances of the loads at the output andisolation ports scale such that energy is conserved, as described above.

Because energy delivered to the isolation port is usually lost, it isusually desirable to synthesize the desired output voltage vector whileminimizing the amplitude of the voltage vector at the Isolation port.One may thus select the power supply levels, amplitudes and phases ofthe two power amplifiers to deliver the desired output while puttingsmall (and preferably the minimum) power into the isolation port.Alternatively, if the signal to be provided at the isolation port is tobe transmitted rather than dissipated, it may be desirable to controlthe supply voltages and PA input amplitudes such that the desiredsignals are synthesized at both the output and Isolation ports whileproviding maximum overall efficiency of delivering energy to the twoports.

FIG. 15 is an IQ plot illustrating achievable output signal ranges for a2-amplifier system where each power amplifier is supplied from one oftwo supply levels. This is illustrated as regions on the IQ diagram interms of the output RF amplitude (phasor length, or RF voltageamplitude). As illustrated, each power amplifier can generate an RFoutput with a maximum amplitude that depends on the power supply levelselected for that amplifier. If both PAs are supplied using a highervoltage L2, an RF output vector anywhere inside the circle labeled L2/L2can be synthesized, with this maximum achieved when the two individualPA vectors are in phase and at their specified maximum amplitudes forthat supply level. If one PA is supplied at the higher voltage level andthe other at the lower voltage level, an RF output vector anywhereinside the circle labeled L2/L1 can be synthesized, with this maximumachieved when the two individual PA vectors are in phase and each attheir specified maximum amplitudes for their supply levels. At thismaximum amplitude, nonzero output is delivered to the Isolation port ofthe combiner, since there is a nonzero difference between the PAoutputs. If both PAs are supplied at the lower voltage level, an RFoutput vector anywhere inside the circle labeled L1/L1 can besynthesized. Lastly, if only one PA, supplied at the lower voltage leveldrives the combiner (e.g., with the drive amplitude of the second PA setto zero and/or the power supply for the second PA set to zero), then avector anywhere inside the circle labeled L1/—can be synthesized. Again,there is nonzero output delivered to the isolation port for this case ifthe combiner is isolating. An additional circle with only one PAsupplied at the higher supply voltage level driving the combiner willalso typically exist. However, operation within this circle will usuallybe less efficient than the other options. The above-described techniquesmay be extended for use within systems having more than two PAs, systemshaving more than two nonzero supply levels, systems using other types ofpower combiners, and/or a combination of the above.

FIG. 16 is an IQ plot Illustrating operation of an Asymmetric MulitlevelOutphasing (AMO) based amplification system that includes two-poweramplifiers and two power supply levels. Each of the power amplifiers maybe operated at a specified maximum output amplitude (with respect to thepower supply voltage used by each amplifier) such that the efficiency ofeach PA can be high. To modulate output power, the two PAs may beoutphased (or phase shifted) such that the sum vector has the desiredamplitude and phase. With an isolating combiner, there may be somedissipation associated with the difference vector sent to the isolationport. By changing the power supply levels depending upon the desiredoutput amplitude, one can keep the amplitude of the difference vectorsmall, maintaining high efficiency.

In at least one embodiment, multiple power amplifiers may be controlledsuch that they synthesize individual output vectors that are in phase,but with amplitudes that may be different (and selecting among differentsupply levels) depending upon desired output. That is, for a givensupply level selection for the two PAs, there is an achievable outputamplitude maximum, which is reached by operating the two PAs at theirachievable maximum. One can reduce the output below this by backing offone or both PAs below their achievable maximum. While backing off thePAs reduces PA efficiency, keeping the PAs in phase keeps the amplitudeat an isolation port small, which can provide enhanced efficiency. Moregenerally, depending on the combiner type, keeping the two PA outputshaving a specified fixed phase relationship provides low isolation portloss and enhanced efficiency. In one approach, the lowest supply levelset may be utilized that will enable the desired output to be achieved,and the drive of one or both PAs will then be backed off to reduce powerfor output amplitudes above that achievable with the next lower supplylevel set. This keeps the PAs operated at or close to their achievablemaximum while minimizing the output at the isolation port. FIG. 17 is anIQ plot illustrating this approach. FIG. 18 is a series of plotsIllustrating how power amplifier drive levels and supply levels may beadjusted to control output power over an output power range inaccordance with an embodiment. As shown, peaks in efficiency may beattained through judicious supply level selection as a function ofpower, and good efficiency may be maintained over a wide output powerrange.

In some embodiments, both backoff and outphasing techniques may becombined in a power amplification system to reduce output amplitudebelow an achievable maximum with a given set of supply voltages, asillustrated in FIG. 15. In this case, for each supply level set andoutput amplitude, a combination of backoff and outphasing can beselected to provide desirable tradeoffs between efficiency andlinearity. Moreover, at very low power, one of the power amplifiers maybe completely turned off and the other amplifier may be operated underbackoff alone. It should also be appreciated that the combination ofoutphasing and backoff of the individual power amplifiers can be used toprovide further control features. For example, instead of a differencevector being delivered to an isolation port to be dissipated, the“isolation port” can be connected to a second output for RF transmission(i.e., for a multi-output system). By controlling both backoff andoutphasing of the two amplifiers, desired outputs can be provided atboth the output port (first output) and the isolation port (secondoutput) while preserving high efficiency.

In some embodiments, multiple different types of power amplifiers, orpower amplifiers optimized for different characteristics, may be usedwithin a power amplification system. The parameters of the system maythen be controlled to best utilize the two types or characteristics ofpower amplifiers. For example, it may be desirable to optimize a firstpower amplifier for operation near its achievable maximum (e.g., aswitched-mode or saturated power amplifier) and a second power amplifierfor operation with good efficiency under a range of backoff (e.g., usinga Doherty power amplifier or chireix outphasing power amplifier). Inthis case, one may control the system such that the first poweramplifier usually operates at or near its achievable maximum, while thesecond regulates the output amplitude by backing off from its maximum.Different output power regions can further be covered by appropriatelyswitching the supply levels of the power amplifier, with the secondpower amplifier operated at the higher of the supply levels if thesupply levels are not the same. At the lowest output levels, the firstpower amplifier may be turned off, and the output controlled only withthe second power amplifier.

In some embodiments, as described above, more than two power amplifiersmay be used to generate a transmit signal in an RF transmitter. Forexample, FIG. 3 is a block diagram illustrating an RF transmitter 50that includes at least four power amplifiers in accordance with anembodiment. As Illustrated, RF transmitter 50 may include: a controller52; first, second, third, and fourth digital to RF modulators 54, 56,58, 60; first second, third, and fourth power amplifiers 62, 64, 66, 68;a power combiner 70; and a voltage control unit 72. As before, one ormore antennas 32 may be coupled to an output of combiner 70. Voltagecontrol unit 72 may provide variable supply voltages V₁(t), V₂(t),V₃(t), V_(n)(t) to first, second, third, and fourth power amplifiers 62,64, 66, 68, respectively, based on one or more control signals fromcontroller 52. First, second, third, and fourth digital to RF modulators54, 56, 58, 60 provide RF Input signals to first, second, third, andfourth power amplifiers 62, 64, 66, 68, respectively based on inputinformation received from controller 52. Combiner 70 combines the outputsignals of first, second, third, and fourth power amplifiers 62, 64, 66,68 to generate an RF transmit signal for delivery to antenna(s) 32.

Controller 52 may use any of the control techniques described above invarious embodiments. In some implementations, controller 52 may use thevoltage control of voltage control unit 72 and the phase and amplitudeinformation delivered to first, second, third, and fourth digital to RFmodulators 54, 56, 58, 60 to ensure that transmit data is accuratelyrepresented within the RF output signal of combiner 70. In addition,controller 52 may use amplitude information delivered to first, second,third, and fourth digital to RF modulators 54, 56, 58, 60 tocontrol/adjust an output power level of combiner 70 (e.g., a transmitpower level, etc.). In some embodiments, this output power controlcapability may be used to provide power backoff for RF transmitter 50.As in previous embodiments, an energy recovery module 30 may be providedto recover energy at an isolation port of combiner 70 when an isolatingcombiner architecture is used.

In at least one embodiment, an RF transmitter may be provided thatincludes a single RF power amplifier. FIG. 4 is a block diagramillustrating an exemplary RF transmitter 80 that includes a single poweramplifier in accordance with an embodiment. As shown, RF transmitter 80includes: a controller 82, a digital to RF modulator 84, a poweramplifier 86, and a voltage control unit 88. The output of poweramplifier 86 may be coupled to one or more antennas 32 to facilitate thetransmission of RF transmit signals to remote wireless entities. Voltagecontrol unit 88 may provide a variable supply voltage V(t) to poweramplifier 86 based on a control signal from controller 82. Voltagecontrol unit 88 may be configured to selectively supply one of aplurality of discrete voltages to power amplifier 86, and may supply thediscrete voltage to the power amplifier via a transition shaping filterin some implementations. The transition shaping filter may, for example,comprise lossless filter elements, including inductors and capacitors,and may further include lossy elements, such as resistors and magneticbeads. The transition shaping filter serves to provide shaping and/orbandwidth limitation of the voltage transitions between discrete levelsand may provide damping of oscillations that might otherwise occur. Thetransition shaping filter may be selected to provide a low-pass filterresponse. Discrete voltage supply levels provided by voltage controlunit 88 may be predetermined or may be adapted over time based onrequired average transmit power levels or other factors.

Digital to RF modulator 84 may provide an RF input signal to poweramplifier 86 based on input Information (e.g., I₁, Q₁) received fromcontroller 82. Controller 82 may use any of the control techniquesdescribed above in various embodiments. In some implementations,controller 82 may use the voltage control of voltage control unit 88 andthe amplitude and phase information delivered to digital to RF modulator84 to ensure that the transmit data is accurately represented within theRF output signal of RF transmitter 80. Controller 82 may use theamplitude Information delivered to digital to RF modulator 84 tocontrol/adjust an output power level of RF transmitter 80 (e.g., atransmit power level). As before, in some implementations, this outputpower control capability may be used to provide power backoff for RFtransmitter 80.

FIG. 4A is a block diagram illustrating an exemplary RF transmitter 80Athat includes a single power amplifier in accordance with an embodiment.The RF transmitter 80A is a specific embodiment of the transmitter 80illustrated in FIG. 4. As shown, RF transmitter 80A includes: acontroller 82A, a digital to RF modulator 84A, a power amplifier 86A,and a voltage control unit 88A. The output of power amplifier 86A may becoupled to one or more antennas 32A. Voltage control unit 80A includes:a supply select unit 241, a multilevel power converter 242, a switchunit 244, and a transition shaping filter 246. Multilevel powerconverter 242 is operative for generating a number of different voltagepotentials (i.e., V₁, V₂, V₃, V₄) on a plurality of correspondingvoltage lines. Although illustrated with four different voltage levels,it should be appreciated that any number of different levels may be usedin different embodiments. Switch unit 244 is capable of controllablycoupling one of the plurality of voltage lines at a time to a powersupply input of power amplifier 86A. Supply select 241 is operative forswitching switch unit 244 between different voltage lines in response toa switch control signal from controller 82A. In at least one embodiment,multilevel power converter 242 may be capable of adapting the voltagevalue on one or more of the voltage lines slowly over time to, forexample, accommodate slow variations in a desired average output power.Transition shaping filter 246 is operative for shaping the transitionsbetween voltage levels within the power amplifier supply voltage signal.In at least one implementation, the transition shaping filter 246 mayinclude a low pass filter.

As described above, in some implementations, a voltage control unit orvoltage control system may include a multi-output (or “multi-level”)power converter to generate multiple voltages from a single inputvoltage. In one approach, a multi-output power converter may beimplemented using a switched-capacitor circuit. A switched capacitorconverter can provide very high power density for synthesizing multipleratiometric voltages from a single voltage. FIG. 4B is a schematicdiagram illustrating an exemplary switched capacitor converter circuit300 in accordance with an implementation. The single input of thismultiple-output converter may be fed directly from a source voltage orbattery 302, or may be fed from another power converter that can adaptthe input voltage to the multiple-output converter (and hence themultiple voltages) over time, including, for example, a buck converter,boost converter, and other magnetics-based converters. In someembodiments, a voltage control unit can operate by switching amongdifferent discrete supply levels and providing this switched supplydirectly to the power amplifier, as illustrated in FIG. 4C.

FIGS. 4D and 4E are schematic diagrams illustrating two switchingnetwork architectures 310, 312 that may be used in power amplifiersystems and RF transmitter systems in accordance with embodiments. Thesetwo architectures 310, 312 are both particularly well suited for use inlow-power systems implemented in CMOS, but may also be used in othersystems. In some implementations, both a switched-capacitor (SC) circuitand a switching network 310, 312 may be realized together on a singleCMOS die. For higher-power systems, it may be advantageous to place acontrol circuit for switching an SC circuit and/or the switching networkon an integrated circuit. It should be appreciated that the switchingnetwork architectures 310, 312 of FIGS. 4D and 4E represent two examplearchitectures that may be used in some embodiments. Other switchingnetwork architectures may be used in other implementations.

As described above, in some embodiments, a supply voltage with switcheddiscrete levels may be provided to a power amplifier through atransition shaping filter. The filter, which may be low-pass in nature,can limit the high frequency content of the PA supply voltage, provideshaping for the voltage transitions between levels, and provide dampingfor the transitions. This filter can also incorporate parasitic elementsassociated with the interconnect of the voltage control unit to the PA,and can be designed such that the frequency content of the supplyvoltage is matched to what may be adequately adjusted for in the PAdrive waveform. FIGS. 4F-4I are schematics illustrating somelevel-transition filter architectures that may be used in variousembodiments. Other filter architectures may alternatively be used.

FIG. 5 is a block diagram illustrating an exemplary RF transmitter 100in accordance with an embodiment. RF transmitter 100 is a specificimplementation of RF transmitter 40 of FIG. 2. As illustrated, RFtransmitter 100 includes: a controller 90; first and second digital toRF modulators 92, 94; first and second power amplifiers 96, 98; avoltage control unit 102; a power combiner 104; and an energy recoverymodule 120. In the illustrated implementation, voltage control unit 102includes: a multi-level power converter 106; first and second switchunits 108, 110; first and second supply selects 112, 114; and first andsecond transition shaping filters 116, 118. Multi-level power converter106 is operative for generating a number of different voltage potentials(i.e., V₁, V₂, V₃, V₄) on a plurality of corresponding voltage lines.Although illustrated with four different voltage levels, it should beappreciated that any number of different levels may be used in differentembodiments. First and second switch units 108, 110 are each capable ofcontrollably coupling one of the plurality of voltage lines at a time toa power supply input of a corresponding power amplifier 96, 98. Firstand second supply selects 112, 114 are operative for switching the firstand second switch units 108, 110 between the different voltage lines inresponse to switch control signals S₁(t), S₂(t) received from controller90. In at least one embodiment, multilevel power converter 106 may becapable of adapting the voltage values on the plurality of voltage linesslowly over time to, for example, accommodate slow variations in adesired average output power. First and second transition shapingfilters 116, 118 are operative for shaping the transitions betweenvoltage levels within each power amplifier supply voltage signal. In atleast one implementation, one or both of the transition shaping fiters116, 118 may include a low pass filter. In some implementations,transition shaping filtration is not used.

As shown in FIG. 5, controller 90 may provide a time varying amplitudevalue and a time varying phase value to each of the digital to RFmodulators 92, 94. That is, controller 90 may provide A_(l)(t), θ₁(t) tofirst digital to RF modulator 92 and A₂(t), θ₂(t) to second digital toRF modulator 94. As described previously, in some implementations, theamplitude and phase Information may be specified using time varying Iand Q data (or some other format), rather than amplitude and phasevalues.

FIG. 6 is a schematic diagram Illustrating an exemplary voltage controlunit 122 in accordance with an embodiment. As illustrated, voltagecontrol unit 122 may include: first and second multi-level powerconverters 123, 124; first and second switch units 126, 128; and firstand second supply selects 130, 132. First multi-level power converter123, first switch unit 126, and first supply select 130 may be used tovary a supply voltage of a first power amplifier of an RF transmitterand second multi-level power converter 124, second switch unit 128, andsecond supply select 132 may be used to vary a supply voltage of asecond power amplifier of the RF transmitter. First and second switchunits 126, 128 and first and second supply selects 130, 132 may operatein substantially the same manner as the corresponding units describedabove.

As Illustrated in FIG. 6, first multi-level power converter 123 maygenerate a first plurality of voltage levels (i.e., V₁, V V₂, V₃, V₄) ona first plurality of voltage lines 134 and second multi-level powerconverter 124 may generate a second plurality of voltage levels (i.e.,V₅, V₆, V₇, V₈) on a second plurality of voltage lines 136. In someembodiments, the voltage levels on the first plurality of voltage lines134 may be the same as the voltage levels on the second plurality ofvoltage lines 136. In other embodiments, they may be different. Asdescribed previously, in some embodiments, different power amplifierdesigns may be used for the multiple different power amplifiers withinan RF transmitter. These different power amplifier designs may requiredifferent discrete voltage levels for optimal performance. In theembodiment depicted in FIG. 6, the same number of voltage levels areprovided for the first and second power amplifier. In some otherembodiments, different numbers of voltage levels may be used fordifferent power amplifiers.

In some implementations, first and second multi-level power converters123, 124 may each be capable of adapting corresponding output voltagelevels over time. As described above, this may be performed to, forexample, accommodate slow variations in a desired average output powerlevel. In at least one implementation, first and second multi-levelpower converters 123, 124 may each include a control Input 138, 140 toreceive a control signal from a corresponding controller (not shown)Indicating when such adaptation is to take place and/or values to usefor the new voltage levels.

FIG. 7 is a schematic diagram illustrating an exemplary voltage controlunit 160 in accordance with an embodiment. As illustrated, voltagecontrol unit 160 may include: first, second, third, and fourth powersupplies 162, 164, 168, 170; first and second switch units 170, 172; andfirst and second supply selects 174, 176. First, second, third, andfourth power supplies 162, 164, 168, 170 may each be operative forgenerating a unique supply potential on a corresponding output line.First and second switch units 170, 172 may each be capable ofcontrollably coupling one of the supply potentials at a time to a powersupply Input of a corresponding power amplifier. First and second supplyselects 174, 176 are operative for switching the first and second switchunits 170, 172 between the different supply potentials in response toswitch control signals S₁(t), S₂(t) received from a correspondingcontroller. In some implementations, first, second, third, and fourthpower supplies 162, 164, 168, 170 may each adapt its output voltagevalue slowly over time to, for example, accommodate slow variations Indesired average output power level. In at least one embodiment, first,second, third, and fourth power supplies 162, 164, 168, 170 eachcomprise a battery or are synthesized from a single source using amulti-output switched capacitor converter. Although illustrated withfour power supplies 162, 164, 168, 170, it should be appreciated thatany number of different power supplies may be used in a particularimplementation.

FIGS. 8 and 9 are flow diagrams Illustrating processes for operating RFtransmitters in accordance with various embodiments.

The rectangular elements (typified by element 182 in FIG. 8) are hereindenoted “processing blocks” and may represent computer softwareinstructions or groups of instructions. It should be noted that the flowdiagrams of FIGS. 8 and 9 represent exemplary embodiments of designsdescribed herein and variations in such diagrams, which generally followthe processes outlined, are considered to be within the scope of theconcepts, systems, and techniques described and claimed herein.

Alternatively, the processing blocks may represent operations performedby functionally equivalent circuits, such as a digital signal processorcircuit, an application specific integrated circuit (ASIC), a fieldprogrammable gate array (FPGA), or conventional electrical or electronicsystems or circuits. Some processing blocks may be manually performed,while other processing blocks may be performed by circuitry and/or oneor more processors. The flow diagrams do not depict the syntax of anyparticular programming language. Rather, the flow diagrams illustratethe functional information one of ordinary skill in the art may requireto fabricate circuits and/or to generate computer software or firmwareto perform the corresponding processing. It should be noted that manyroutine program elements, such as initialization of loops and variablesand the use of temporary variables, are not shown. It will beappreciated by those of ordinary skill in the art that unless otherwiseindicated herein, the particular sequences described are illustrativeonly and can be varied without departing from the spirit of the conceptsdescribed and/or claimed herein. Thus, unless otherwise stated, theprocesses described below are unordered meaning that, when possible, thesequences shown in FIGS. 8 and 9 can be performed in any convenient ordesirable order.

Referring now to FIG. 8, a method 180 for operating an RF transmitterhaving at least two digital-to-RF modulators driving at least two poweramplifiers will be described. First, transmit data may be obtained thatis intended for transmission to one or more remote wireless entities(block 182). The transmit data may be used to provide input informationfor the at least two digital-to-RF modulators to control amplitudes andphases of output signals of the modulators (block 184). The inputinformation generated for the at least two digital-to-RF modulators maybe different for different modulators. The output signals of the atleast two digital-to-RF modulators may then be delivered tocorresponding inputs of the at least two power amplifiers of the RFtransmitter (block 186).

The transmit data may also be used to select supply voltages for each ofthe at least two power amplifiers (block 188). The output signals of theat least two power amplifiers may be combined to generate a transmitteroutput signal for the RF transmitter (block 190). Any of the controltechniques described herein may be used to generate the inputinformation for the at least two digital-to-RF modulators and to selectthe supply voltages for the at least two power amplifiers. Thetransmitter output signal may be delivered to one or more antennas to betransmitted into a wireless channel (block 192). In at least oneimplementation, the input information generated for each digital-to-RFmodulator may be representative of both an amplitude value and a phasevalue. The amplitude value and the phase value will typically vary withtime (i.e., they can change from sample to sample). In at least oneimplementation, I and Q values may be provided for each of the at leasttwo digital-to-RF modulators. In some other implementations, timevarying amplitude and phase values may be provided. As described above,different input information may be generated for each of the at leasttwo digital-to-RF modulators.

In at least one approach, the amplitude and phase values provided forthe at least two digital-to-RF modulators and the supply voltagesselected for the at least two power amplifiers may be selected in amanner that results in an accurate representation of the transmit datawithin the RF output signal of the RF transmitter. The amplitude valuesgenerated for the at least two digital-to-RF modulators may be selectedto control or adjust an output power level of the RF transmitter in someembodiments. In one approach, the amplitude values may be selected toachieve a desired level of power backoff within the RF transmitter.

FIG. 9 is a flow diagram Illustrating a method 200 for operating an RFtransmitter having a single digital-to-RF modulator and a single poweramplifier. Transmit data is first obtained that is intended fortransmission to one or more remote wireless entities (block 202). Thetransmit data may be used to generate input information for thedigital-to-RF modulator to control an amplitude and phase of an outputsignal of the modulator (block 204). The amplitude of the output signalof the modulator may be controlled in order to achieve a desiredtransmit power level for the RF transmitter. The output signal of thedigital-to-RF modulator may be delivered to a corresponding input of thepower amplifier of the RF transmitter (block 206).

The transmit data may also be used to select a supply voltage for thepower amplifier (block 208). Any of the control techniques describedherein may be used to generate the input Information for the singledigital-to-RF modulator and to select the supply voltage for the singlepower amplifier. The output signal of the power amplifier may be coupledto one or more antennas for transmission into a wireless channel (block210). In at least one implementation, the input information delivered tothe digital-to-RF modulator may be representative of both an amplitudevalue and a phase value. In at least one approach, the phase valuegenerated for the digital-to-RF modulator and the supply voltageselected for the power amplifier may be selected in a manner thatprovides an accurate representation of the transmit data at an output ofthe RF transmitter. The amplitude value generated for the digital-to-RFmodulator may be selected to control or adjust an output power level ofthe RF transmitter. In one approach, this power control capability maybe used to achieve a desired level of power backoff for the RFtransmitter.

FIG. 11 is a flow diagram illustrating a method 230 for selectingvoltage levels for one or more power amplifiers of a power amplificationsystem based on a window of samples in accordance with an embodiment. Asdescribed previously, FIG. 10 is a diagram Illustrating such a samplewindow 220. The method 230 may be used, for example, within the methods180, 200 of FIGS. 8 and 9 (e.g., in block 188 and block 208,respectively). As shown in FIG. 11, it may first be determined whetherN_(W) current and future samples can use a lower set of voltage levelsthan a previous sample and still meet output power requirements (block232). If so (block 232-Y), then a new set of voltage levels may beselected that are as low as possible (or at least lower than theprevious sample) to support all N_(W) current and future samples (block234). If the N_(W) samples cannot use a lower set of voltage levels thanthe previous sample, it may next be determined whether the set ofvoltage levels used for the previous sample is sufficient to support thecurrent sample (block 236). If not (block 236-N), then a new set ofvoltage levels may be selected that are as low as possible to supportthe current sample (block 238). If the set of voltage levels used forthe previous sample is sufficient to support the current sample (block236-Y), then the same level selection may be used for the current samplethat was used for the previous sample. This process may be repeated foreach new sample.

In the description above, various concepts, circuits, and techniques arediscussed in the context of RF transmitters that are operative fortransmitting signals via a wireless medium. It should be appreciatedthat these concepts, circuits, and techniques also have application inother contexts. For example, in some implementations, features describedherein may be implemented within transmitters or drivers for use inwireline communications. In some other implementations, featuresdescribed herein may be implemented within other types of systems thatrequire highly efficient and highly linear power amplification for datacarrying signals. In other implementations, features described hereinmay be applied to systems that require linear power amplification foraudio applications, for ultrasonic applications, and in RF poweramplification for heating, industrial processing, imaging, and RF powerdelivery.

Having described exemplary embodiments of the invention, it will nowbecome apparent to one of ordinary skill in the art that otherembodiments incorporating their concepts may also be used. Theembodiments contained herein should not be limited to disclosedembodiments but rather should be limited only by the spirit and scope ofthe appended claims. All publications and references cited herein areexpressly incorporated herein by reference in their entirety.

What is claimed is:
 1. A radio frequency (RF) power amplifier system,comprising: an RF power amplifier; a digital-to-RF modulator to generatean RF Input signal for the RF power amplifier based on input informationindicative of amplitude values and phase values; a voltage control unitto provide a variable supply voltage to the RF power amplifier inresponse to a control signal, the variable supply voltage being selectedfrom a plurality of discrete voltage levels; and a controller to providethe input Information to the digital-to-RF modulator and the controlsignal to the voltage control unit based, at least in part, on data tobe transmitted from the RF transmitter, the controller being configuredto make decisions about voltage level changes for the RF power amplifierbased, at least in part, on a window of data samples representing datato be transmitted by the RF transmitter.
 2. The RF power amplifiersystem of claim 1, wherein: the controller is configured to selectamplitude and phase values for the digital-to-RF modulator and voltagevalues for the RF power amplifier to generate an accurate representationof the transmit data within an output signal of the RF power amplifiersystem.
 3. The RF power amplifier system of claim 1, wherein thecontroller is configured to select a discrete voltage level for acurrent data sample based, at least in part, on a voltage level selectedfor at least one previous sample, the current data sample, and at leastone future data sample.
 4. The RF power amplifier system of claim 3,wherein the voltage control unit includes a transition shaping filter tofilter a voltage signal applied to the power amplifier.
 5. The RF poweramplifier system of claim 1, wherein: the controller is configured tomake voltage level change decisions for the power amplifier in a mannerthat achieves efficient power amplifier operation with a reduced numberof voltage level transitions.
 6. The RF power amplifier system of claim1, wherein: the controller is capable of adapting the length of thewindow of data samples over time based on a predetermined amplifierperformance criterion.
 7. The RF power amplifier system of claim 1,wherein: the window of data samples has a fixed length.
 8. The RF poweramplifier system of claim 1, wherein: the controller is configured toadapt one or more values of the plurality of discrete voltage levelsused by the voltage control unit over time.
 9. The RF power amplifiersystem of claim 1, wherein, to determine a voltage level for use by thepower amplifier for a current sample, the controller is configured to:if all samples in the window of data samples can use a lower voltagelevel than was used for a previous sample to achieve a desired outputpower, select a new voltage level that is lower than that used for theprevious sample for use with the current sample.
 10. The RF poweramplifier system of claim 9, wherein, to determine a voltage level foruse by the power amplifier for a current sample, the controller isfurther configured to: if all samples in the window of data samplescannot use a lower voltage level than the previous sample to achieve adesired output power: if the voltage level used for the previous sampleis sufficient for use with the current sample, use the voltage levelused for the previous sample for the current sample; and if the voltagelevel used for the previous sample is not sufficient for use with thecurrent sample, select a new voltage level for the current sample. 11.The RF power amplifier system of claim 1, wherein: the controller iscapable of selecting a voltage value for the power amplifier that islower than required to generate a desired instantaneous output powerlevel if the need for a higher voltage level is short in durationcompared to the time duration of the window of data samples.
 12. The RFpower amplifier system of claim 1, wherein: the controller is capable ofselecting a voltage value for the power amplifier that is higher than aminimum voltage sufficient to generate a desired instantaneous outputpower level if the duration of time for which a lower voltage level issufficient is short compared to the time duration of the window of datasamples.
 13. The RF power amplifier system of claim 1, wherein: the RFpower amplifier is a first RF power amplifier; and the RF poweramplifier system further comprises: at least one additional RF poweramplifier; and a combiner to combine the output signals of the first RFpower amplifier and the at least one additional RF power amplifier. 14.A machine implemented method for operating an RF transmitter having adigital-to-radio frequency (RF) modulator driving a power amplifier, themethod comprising: obtaining transmit data to be transmitted from the RFtransmitter, providing input Information for the digital-to-RF modulatorbased, at least in part, on the transmit data, the input information tocontrol an amplitude and a phase of an RF output signal of thedigital-to-RF modulator; and selecting a supply voltage for the poweramplifier based, at least in part, on the transmit data, whereinselecting a supply voltage includes making decisions about voltage levelchanges for the RF power amplifier based, at least in part, on a windowof data samples representing data to be transmitted by the RFtransmitter.
 15. The method of claim 14, further comprising: deliveringan output signal of the power amplifier to one or more antennas fortransmission.
 16. The method of claim 14, further comprising selecting adiscrete supply voltage level for a current data sample based, at leastin part, on a voltage level selected for at least one previous sample,the current data sample, and at least one future data sample.
 17. Themethod of claim 16, wherein: the amplitude of the RF output signal ofthe digital-to-RF modulator and the selection of the discrete supplyvoltage level are controlled to achieve a desired instantaneous outputpower of the RF transmitter.
 18. The method of claim 14, whereinselecting a supply voltage for the power amplifier comprises: if allsamples in the window of samples can use a lower voltage level than aprevious sample to achieve a desired output power, selecting a newvoltage level that is lower than that used for the previous sample. 19.The method of claim 18, wherein selecting a supply voltage for the poweramplifier further comprises: If all samples in the window of samplescannot use a lower voltage level than the previous sample to achieve thedesired output power: if the voltage level used to support the previoussample is sufficient for use with a current sample, continue using thevoltage level used to support the previous sample for the currentsample; and if the voltage level used to support the previous sample isnot sufficient for use with the current sample, selecting a new voltagelevel that is sufficient for use with the current sample.
 20. The methodof claim 18, wherein: selecting a new voltage level that is lower thanthat used for the previous sample to support all samples includesselecting a new voltage level that is as low as possible and stillcapable of generating desired instantaneous output power levels for allsamples in the window.
 21. The method of claim 18, wherein: selecting anew voltage level that is sufficient for use with the current sampleincludes selecting a new voltage level that is as low as possible togenerate a desired instantaneous output power level for the currentsample.
 22. An apparatus comprising a computer readable medium havinginstructions stored thereon that, when executed by a computer, perform amethod for operating an RF transmitter having a digital-to-radiofrequency (RF) modulator driving a power amplifier, the methodcomprising: obtaining transmit data to be transmitted from the RFtransmitter; providing input Information for the digital-to-RF modulatorbased, at least in part, on the transmit data, the input information tocontrol an amplitude and a phase of an RF output signal of thedigital-to-RF modulator; and selecting a supply voltage for the poweramplifier based, at least in part, on the transmit data, the supplyvoltage being selected from a plurality of discrete voltage levels,wherein selecting a supply voltage includes making a decision aboutsupply voltage level for the RF power amplifier based, at least in part,on a window of data samples representing data to be transmitted by theRF transmitter.
 23. The apparatus of claim 22, wherein: selecting asupply voltage for the power amplifier includes selecting a discretesupply voltage level for a data sample based, at least in part, on thedata sample, at least one subsequent data sample, and a voltage levelselected for at least one previous sample.
 24. The apparatus of claim22, wherein: providing input information for the digital-to-RF modulatorincludes providing input information that controls an amplitude of an RFoutput signal of the digital-to-RF modulator in a manner that achieves adesired power level at an output of the RF transmitter.
 25. Theapparatus of claim 22, wherein: selecting a supply voltage for the poweramplifier is performed in a manner that achieves efficient poweramplifier operation with a reduced number of voltage level transitions.26. The apparatus of claim 22, wherein the method further comprises:adapting a length of the window of data samples over time based on apredetermined criterion.
 27. The apparatus of claim 22, wherein: thewindow of data samples has a fixed length.
 28. A machine implementedmethod for selecting a voltage level for a power amplifier within aradio frequency (RF) transmitter based on input data to be transmitted,the voltage level being selected from a plurality of discrete voltagelevels, the method comprising: Identifying a window of samples to betransmitted from the transmitter; and if all samples in the window ofsamples can use a lower voltage level than a previous sample to achievea desired instantaneous output power, selecting a new voltage level thatis lower than that used for the previous sample.
 29. The machineimplemented method of claim 28 further comprising: if all samples in thewindow cannot use a lower voltage level than the previous sample toachieve the desired output power: if the voltage level used for theprevious sample is sufficient for use with a current sample, using thevoltage level used for the previous sample for the current sample; andif the voltage level used for the previous sample is not sufficient foruse with the current sample, selecting a new voltage level that issufficient for use with the current sample.
 30. The method of claim 28,wherein: selecting a new voltage level that is lower than that used forthe previous sample includes selecting a new voltage level that is aslow as possible to support all samples in the window.
 31. The method ofclaim 28, wherein: selecting a new voltage level that is sufficient foruse with the current sample includes selecting a new voltage level thatis as low as possible for use with the current sample.